Tone producing and amplifier control for electronic organs



May 29 1951 F. M. SCHMIDT 2,555,295

TONE` PRODUCING AND AMPLIFIER CONTROL FOR ELECTRONIC ORGANS Flled Jan.8, 1948 2 Sheets-Sheet l (Ehh hl l 2 Sheets-Sheet 2 W Q INVENTOR. w 5L/5yd/wt FOR ELECTRONIC I ORGANS F. M. SCHMIDT TONE PRODUCING ANDAMPLIFIER CONTROL www www s@ MG May 29, 1951 Filed Jan. 8, 1948 PatentedMay 2.9, 1951 UNITED STATES PATENT OFFICE'.

TONE PRODUCING ANDYV AMPLIFIER ooN- TROL FOR ELECTRONIC ORGAN S FrancisM. Schmidt, North Tonawanda, N. Y., as-

signor to The Rudolph Wurlitzer Company, North Tonawanda, N. Y., acorporation of Ohio Application January 8, 1948, Serial o. 1,114

Claims'.

v:Ellis,invention relates to tone producing systems v,for electronicmusical instruments, and s peeicjally electronic organs; and concernsparticularly the amplifier circuits of such tone producing, systems, andtheir associated elements.

In' the operation of electrical musical instruments, and' specificallyelectronic organs of the typeto which the present invention particularlyrelates, there is provided a plurality of banks of tne generatorslwh'ichare selectively operable to g e, r 1 eratf a v tones in the form ofelectrical waves, the Waves beingcombined'and transmitted to a loud's1eaker or other reproducing apparatus 1e1n` they' are transformed andemitted' as so nfd. In such'l instruments it is desirable that ap1ifyihg meanS be provided, for amplifying an "transmitting thepulsations or oscillations produced by the tone generators into sound orEItIiicaIpUISatIIS of Suitable intensity, and that in ans be`V providedfor varying such intens'ityatwill and" in accordance with operatingrequirements. It is furthermore desirable, that in" the amplification ofthe pulsations fromY theV tne'g'en'erators, the quality o r wave formthereeffin son'instances be preserved and in otherv instancesv reshaped,as may be required; and that' nean'sbe provided for accentuating oratt'e'niiating the'various'tone and harmonic component frequencies inrespect to each other, eitlfler` at a' iiked'volume level, or atvariable volurne levelsV throughout the range of the instru- It'i'sanobject of the present invention to providf'a'n improved tone producingand amplifier Circuitfor electrical musical' instruments, and 'f lspecicall'y to' provide an improved tone produc" g andl variable ampliercircuit for an electronic' organ of the typev defined.

Itis afurtlier object of the present invention toprovide' animprovedamplier structure specifically 'for electronic'organs, wherein the'quality' orwave shape of the electric oscillations produced from thetone generators is selectively pesrvedlor variedjin'a controlled andimproved manner, so'asto provide desired tonal effects.

`Afstill"f u'rtherobject' of the present invention isto provide, in anamplifier structure for electro lo organs ofjthe type defined', animproved it controlincorporatingmeans for attenuatingj orf accentuatingthe diierent frequencies oifftlie variousr tones produced, at thevarious tone s, jso"va's'f to provide' proper tonal effects'throughoutthe/instrument volume range.

n notherwobject is v to provide an improved amplifier swelllc'ontrol"for-` electronic organs;

Various other objects, advantages, and features of the invention will beapparent from the following speci'cation when taken in connection withthe accompanying drawings wherein a prefer-red: embodiment is set forthfor purposes of illustration.

In the drawings wherein like reference numorals refer to like partsthroughout:

Fig 1 is a diagrammatic View of an electrical musical instrument, andspecifically an electronic organ, incorporating a tone producing andampliiier circuit constructed in accordance with andembodying theprinciples of the invention;r

Fig. 2' is a detail view showing a part of the tone'genera-tors;

lig. 3 is a detail View of the volume or' swell pedal; and.

Figa is an electric circuit diagram illustrating the control structuresof the present invention.

The particular electronic organ illustrated in Fig. 1 comprises a casingor housing lli arranged to support a swell manual or keyboard I2, agreat manual or keyboard.` M, anda pedal clavier IB; There is alsoprovided a volume control pedal i8, by means of which the overall volumeor levely of the' instrument may be controlled, and a series of stoptablets 20 by means of which thevarious tone generators are caused tobe'ac'tivated and thei volume thereof controlled, inbanks, intheparticular instrument shown. Further disposed within the casing lil is aplurality of banks of tone generators 2 2 which may, for example, be ofthe type and `kind shown in the patent to Hoschke, 2,015,014. Morespecifically, and as shown in Fig. 2, these tone generators comprise aseries of vibratory reeds 24 individually disposed within cells 26, eachreed having associated therewith an adjustable pickup screw 2`8 formin gwith its associated reed a capacitor variable in accordance withthereedvibrations.

`Air is admitted to the cells 26, so as to cause the selective vibrationofA the' reeds, by means of a series'v of pallet Valves 3i), theselective operaftion of whichis controlled conjointly by thekeys of themanuals, pedal clavier, and the stops 20. rIfhi's cor 1troll is effectedby coupler boards as shown at 32and 34, the coupler boards contain'- ingswitches selectively actuated by theV keys of thejman'uals and pedalclavier, in accordance withy the selected stops which havev beendepressed, so as to cause theoperation' ofthe desired pallet valves tothereby admit actuating airz' to vibrate thedesired reeds;

The air supply for the reeds is provided froma' 3 blower 36. Theelectrical output from the tone generators or reeds is transmitted to anamplifier 38, the details of which form a part of the present invention,and thence tc further amplifying means as indicated at 40, and to aseries of speakers cr tone outlets 42, Fig. 4.

Certain of the stops 20 effect a selection of the tone generators foroperation, as previously described, whereas other of the stops areprovided for intensity control purposes to selectively control theintensity or level of the individual banks of tone generators. Thisintensity control, for the individual banks of reeds, is moreparticularly set forth in my application, Serial Number 1,112, filed ofeven date herewith, and insofar as such control forms a part of thepresent invention is diagrammatically illustrated in Fig. 4, theintensity control switches being indicated therein by the referencenumerals 44, 45 and 48.

Referring to Fig. 4, it will be seen that there is provided a full waveelectronic tube rectier unit, generally indicated by the numeral 50,powered from a suitable source of A. C. supply, as indicated at 52. Thenegative D. C. return line from the rectifier is indicated at 54,connected to ground through a resistor 5S. The positive D. C. outputwire `58 from the rectiiier is also connected to ground through astabilizing resistor 60, and connects through a ripple iilter comprisingan iron core choke 62 and a pair of capacitors 84 to a line 68 which inturn connects with a line 68 leading to the intensity controlarrangement, previously mentioned.

More particularly, this intensity control structure comprises a pair offixed resistors 'I0 and 'I2 by means of which the voltage of supply wire68 is communicated, respectively, and at the desired value, to a wire'i4 leading to a bank of celeste reeds 1G, and a wire I8 leading to abank of diapason reeds 80. The unit further comprises a series of threeadjustable resistors 82, 84 and 86 for applying selected reducedvoltages to the viole reeds 88 by means of a wire 90, to the pedal reeds92 by means of a wire 94, and to the flute reeds 96 by means of a wire98. Stop actuated justable resistor 84 and applies a predeterminedgreater voltage to the pedal reeds determined by the value of theresistor |02. Stop switch 48, when closed, shorts the adjustableresistor 86 and applies a predetermined greater voltage to the flutereeds determined by the value of a resistor |04.

The several banks of reeds are preferably interconnected in the detailedmanner illustrated in my co-pending application, Serial Number 1,113,also filed of even date herewith. As diagrammatically illustrated inFigure 4, it will be seen that each reed bank is connected to groundthrough a resistor |06 and a by-pass condenser as indicated at |08, thereeds being connected in parallel to the resistor and condenser, as bestshown in Fig. 2.

As also best shown in Fig. 2, for the celeste reeds, the adjustable tonescrews 28 are connected in parallel to a wire such as indicated at IIO,which wire, Fig. 4, leads to an amplifier structure presently to bedescribed. Similarly, the diapason tone screws and a portion of theviole tone screws are connected in parallel to a wire as shown at I I2;the remaining portion of the viole reeds are connected in parallel to awire I I4, and

4 the pedal and flute tone screws are connected in parallel and incommon to a wire IIS, all leading to the amplifier structures.

As more particularly pointed out in said Hoschke patent, the vibrationsof the reeds, as the reeds are selectively vibrated by the air blasts,produce pulsations in the output wires |I0, H2, I|4 and IIB, leadingfrom the tone screws, due to the Variable capacitor action between thevibrating reeds and the tone screws. The capacitors |08 form a part ofthis signal circuit. The intensity of the pulsations is determined bythe intensity of the polarizing voltage applied to the reeds undercontrol of the various resistors 'I0- 12, B2-86, and I00-|04, previouslydescribed.

The character of the sounds, or more accurately stated, the wave shapeof the electrical pulsations produced by the motion of the reeds, isqualitatively controlled to provide the desired and qualitativelydifferent tonal eifects from the various banks of reeds, as indicated bythe designations applied thereto. These qualitative characteristicsresult and are imparted in part by the physical components of the tonescrewreed relationships; and in accordance with the present inventionare further preserved, modiiied and controlled in accordance with thetonal characteristics desired, by the treatment accorded the electricalpulsations within the amplifier structure.

More particularly, referring to Fig. 4, it will be seen that line |l2from the diapason reeds, connects through a resistor I I 8 to line |20leading to the grid of a class A straight line amplier triode |22.Similarly, line IIS, leading from the tone screws of the flute reeds isconnected through a pair of resistors |24 also connected to the line |20and to the grid of tube |22. Line |I2 is connected to ground, in advanceof resistor IIB, through a resistor as indicated at |26. The values ofresistors IIS, |24 and |28 are so selected as to impart the desiredvoltages from the reeds to the tube grid; and further cooperate in theirresistor values with the capacitance values of the tone screw-reedcapacitors and the associated wiring connections, providing shapingcircuits for imparting the desired wave shape or tone quality to thegrid of the amplier tube.

Whereas the output from the flute, pedal, diapason, and a part of theviole reeds is transmitted to a straight line amplifier channelcomprising the tube |22, the output from the celeste reeds, and from theremainder of the viole reeds is transmitted by wires IIB and lI-l to acommon wire |20 which leads through a resistor |30 to the grid of aclass A amplifier triode |32 forming a part of an attenuating or phasediscriminating wave shaping amplifier circuit; said tube circuit beingso constituted by reason of its plate circuit connections presently tobe described. Wire |28 is connected to ground through a grid resistor asindicated at |34.

As in the case of the resistors for the tube |22 previously described,resistors |30 and |34 in effect form a voltage divider forpredetermining the grid applied voltage from the reeds; and further areso selected that their resistor values combine with the capacitancevalues of the tone generators and associated circuits to impart adesired shaping to the wave form applied to the tube grid. Theseresistors furthermore serve as isolation resistors for the highimpedance tone generators.

The cathode bias resistor for the tube |32, and

the associated by-pas capacitor, are conventionally indicated in Fig. 4at |36 and |38. Simil'arly,` the cathode bias resistor and by-passcapacitor with the tube |22 are indicated respectively at |40 and |42.

The plate load circuit for the tube |22 includes a pair of resistors |44and |46 connected to the B+ voltage supply line 58, through a furthervoltage dropping resistor |41, said resistors forming a voltage divider,the intermediate portion of which is connected through a couplingcapacitor |46 and a, resistor |50 to the control grid of a pentodeamplifier tube, as shown at |52. Resistor |41 also forms with capacitor|53 a further ripple iilter for the B+ supply from the rectifier.

The plate of tube |32 is connected to the control grid of amplifierpentode |52 through a load circuit which includes a resistor |54interconnecting the plate of tube |32 to the B+ supply, and a shapingcircuit which includes a, coupling capacitor |56, a choke |58, and aresistor |60 leading to ground, an intermediate portion of the chokebeing connected by means of a capacitor |62 and a resistor |80 to thecontrol grid of the pentode ampliiier. The resistance, capacitance, andinductance values of the foregoing elements are proportioned to providea shaping circuit of predetermined resonance characteristics for theoutput signal from the ampliiier tube |32 so as to attenuate the signaltones and the harmonic components thereof in accordance with theirfrequency, whereby to effect a selective shaping of the tones or signalsemanating from the reeds interconnected with tube |32, as compared withthose connected to and amplified by the ampliiier |22.

The control grid of amplifier tube |52 is connected to ground through anisolation and grid load resistor |65 and a resistor |68, which latterresistor together with the resistors and |12 forms a voltage divider fordetermining the applied grid voltage of the amplifier pentode.

More particularly, and as further shown in Fig. 4, these elements form apart of the swell or volume control circuit for the amplifier. It willbe seen that the negative line 54 of the rectier is normally maintainedat a, predetermined value below ground potential through the action ofresistor 56. Wire 54 is connected through a ripple filter composed of achoke |14 and a capacitor |16, and through a current limiting resistorV58 to a wire |80, which latter wire is connected to ground through avariable resistor |82 adapted for operation by the swell or volumecontrol pedal I8 of the instrument, as previously described.

As shown for example in Fig. 3, the swell or volume control pedal may besuitably pivotally mounted, as indicated at |84, and arranged uponpivotal operation to reciprocate a rack |80 in geared engagement with apinion |88. This latter pinion is arranged by means of a suitable beltand pulley structure |90 to actuate a rotary tap resistor |82, therotary arm of which is connected to ground and the resistor coil ofwhich is connected at one end to the wire |80. It will be seen that asthe pedal is actuated the resistor will be progressively short circuitedwhereby to vary the Voltage level of the Wire |80 from ground, when theresistor is completely short circuited, progressively to its maximumnegative 'bias at which it is normally maintained by reason of theaction of resistor 56.

'Line |80V is connected through a further iilter unit comprising anisolating resistor |94, a choke |98 and a capacitor |98 to a wire 200connected to the voltage divider Hi8-|12, previously described. As thevariable resistor |82 is varied, the voltage applied to the control gridof the amplier pentode |52 will be correspondingly adjusted, whereby toVary the amplifier gain and control the volume level of the instrument.

he screen grid connections for tube |52 include a screen droppingresistor 202 and a screen cy-pass condenser 204. The cathode andsuppressor grid connections include a pair of voltage dividing resistors206 and 208, the intermediate point of which is connected in common tothe tube cathode and suppressor grid connections. It will be noted thatthe resistor 208 is not bypassed which speeds up the response of theamplifier tube and also provides a predetermined `measure ofdegeneration.

The plate circuit for the amplifier pentode includes a plate loadresistor 2|0 connected to the B+ supply, and a further 'shaping circuitcomprised of a pair of series connected resistors 2|2 and 2|4, acondenser 2|6 in by-pass relation to the resistor 2|2, a condenser 2|8in series connection with the resistor 2|4, and a variable resistortriode 220, the function of which will now be described.

More particularly, it will be seen that the grid of the variableresistor tube is connected by means of a grid load resistor 222 to thevoltage control line 200 for the control grid of the main ampliiierpentode. This connection is made between the resistors |10 and |12, in aparticular embodiment shown, and the circuit further includes a pair offilter capacitors 224 and 226.

It will be seen that as the voltage level of line 2 c is varied, aspreviously described, to vary the gain of the pentode amplifier tube|52, the applied current voltage of the variable resistor tube 220 willbe correspondingly adjusted, whereby to vary the resistance of the platecircuit afforded through the tube 220, this tube being connected toground through line 228 as shown.

It will thus be seen that as the gain of the amplier |52 is varied, theresistance value of the shaping circuit comprised of the elements2|2-22s is correspondingly changed as the resistance value of tube 220is adjusted, whereby a progressively different shaping circuit isproduced at each volume level. By this means a different frequencyattenuation or frequency discrimination is obtained in the ampliiiercircuit, at each volume level, to produce desired tone qualitiesthroughout the volume level range.

The plate load circuit for the ampliier |52 further includes a couplingcapacitor 230, a grid load resistor '232, and a series-connected currentlimiting resistor 231i leading to the grid of an amplifier triode 236which may conveniently be arranged within a common envelope with thevariable resistor tube The cathode bias connections for the amplifiertriode 255 are conventionally indicated at 238 and 258, respectively.

The load plate circuit for the amplifier tube 235 includes a loadresistor 242, connected to the B+ supply, and a pair of seiies-connectedresistors 244 and 246, the intermediate portion of which is connected bymeans of a coupling capacitor 248, a grid load resistor 250, and aseriesconnected current limiting resistor 252 with the control grid of apower amplier tube 254.

It'will be seen that the resistor 245 is connected to ground through afixed capacitor 256 and a variable capacitor 258 formed of vthreecapacitor elements, any one of which may be selectively connected toground through the action of an adjustable tap switch connection 260.The fixed capacitor 255, the adjustable capacitor 253, and theassociated resistors 244 and 246 comprise a further shaping or frequencydiscriminating circuit for the tone signals, the capacitance values andresultingly the effective operation of which may be selectivelycontrolled by adjustment of the tap switch 260. The adjustable capacitorstructure 258 is designed for cooperation with the volume pedal control182, previously described, and may be aptly termed a pedal compensator.in operation, the adjustable capacitor 258 is provided primarily forcompensation of lower frequencies, and therefore also may be termed abass compensator structure. The adjustable tap switch 28D may bemanually preset to adapt the instrument to the acoustics of the locationwithin which the instrument is to be played.

The power amplifier 254 is preferably a beam power tube connected foroperation as a triode, the suppressor grid and cathode being connectedin common to a 'pair of cathode bias resistors 262 and a by-passcapacitor 264, as shown. The screen grid and plate are connected incommon to the primary coil 266 of a load transformer 268, the secondarycoil 210 of which is arranged for connection to the further amplifier'and speaker structures 40 and 42, previously described. Theseconnections are diagrammatically indicated herein and are more fully setforth in my co-pending application Serial lNumber 1,115, of even dateherewith.

The load circuit for power tube 254 further includes a plate voltagedropping resistor 2l2 by means of which the plate circuit isinterconnected with the B-isupply, and a by-pass capacitor as indicatedat 2'4 for the signal circuit.

It will be seen that the tone producing and amplifler circuit herein setforth provides a plurality of series of tone generators, the volume ofwhich may be selectively controlled by controlling the polarizingvoltage applied thereto. The qualitative output from the tonegenerators, provided by the physical relationships thereof, is preservedor modified in accordance with desired requirements by the selectiveshaping treatment accorded within the amplier structures. An improvedand simplified gain control is provided for the amplifier, and means isprovided automatically operable therewith for varying the frequencydiscriminating or wave shaping characteristics of the signal circuit asthe sound level of the amplifier is varied, this action being 'effectedthrough the variable resistor tube 228. iMeans is further provided, bythe pedal or bass .compensator 258, for facilitating the selective:adjustment of the frequency discriminating characteristics of thecircuit, in accordance with the acoustics of the location within whichthe instrument is to be played.

It is obvious that various changes may be made in the particularelectronic musical instrument, and in the specific embodiment thereofset forth for purposes of illustration, without departing from thespirit of the invention. The invention is accordingly not to be limitedto the specific structures shown and described, but only as limited bythe following claims.

The invention is hereby claimed as follows:

l. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interposed between said tone'generators andoutlet including a plurality of amplifier channels individuallyconnected to said sets of tone generators said amplifier channelsincluding linear amplifying means and frequency discriminatingamplifying means of predetermined different value, connectedsubstantially in parallel therewith.

2. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, means interposed between said tone generators and outletincluding a plurality of ampliiier channels individually connected tosaid sets of tone generators, one of said channels providing straightline amplification, and another of said channels forming a frequencydiscriminating amplifler circuit, means connecting some of said tonegenerators to said first mentioned channel means connecting some of saidtone generators to said second mentioned channel, and of said tonegenerators some means connecting to both of said channels.

3. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interposed between said tone generators andoutlet including a plurality of amplifier channels individuallyconnected to said sets of tone generators, at least one of saidamplifier channels including frequency discriminating means ofpredetermined different value and at least one of said channels beingsubstantially linear, a combining amplifier channel fed by saidplurality of amplifier channels, adjustable frequency discriminatingmeans in said combining amplier channel, and means for controlling thegain of said combining ampliiier channel.

4. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators and outletincluding a plurality of ampliiier channels individually connected tosaid sets of tone generators, said amplifier channels includingfrequency discriminating means of predetermined different value, acombining amplifier channel fed by said plurality of amplifier channels,means for controlling the gain of the combining amplifier channel, andfrequency discriminating means connected to said combining ampliiierchannel and to said gain control means thereby being adjustable inaccordance with the operation of the gain control means.

5. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interposed between said tone generators andoutlet including a plurality of amplifier channels individuallyconnected to said sets of tone generators, said amplifier channelsincluding frequency discriminating means of predetermined differentvalue, a combining ampliiier channel fed by said plurality of amplifierchannels, means for controlling the gain of the combining amplifierchannel, and manually adjustable frequency discriminating meansconnected to said combining amplifier channel for controlling theoperation thereof.

6. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators and outletincluding a plurality of amplifier channels individually connected tosaid sets of tone generators, said amplifier channels includingfrequency discriminating means of pre determined different value, acombining amplifier channel fed by said plurality of amplier channels,means for controlling the gain of the combining amplifier channel, and aplurality of frequency discriminating means connected to the combiningamplifier channel, one of said irequency discriminating means beingmanually adjustable, and the other of said frequency discriminatingmeans being responsive to the operation of said gain control means.

7. An electronic musical instrument having a sound producing systemcomprising` a plurality of sets of tone generators of diiferent quality,a tone outlet, and means interposed between said tone 'generators andoutlet including a plurality of amplifier tubes individually connectedto said sets of tone generators, a plurality of load circuits ofpredetermined different reactance individually provided for said tubes,an amplifier tube connected in common to said load circuits, meansincluding a manually adjustable control member for controlling the gainof said last mentioned tube, and variable frequency discriminating meansconnected to the output circuit of said tube.

8. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interposed between said tone generators andoutlet including a common amplifier tube having a cathode, control grid,and plate circuit, a manually operable control member for adjusting therelative potential of the grid and cathode, to thereby control the gainof said tube, and a plurality of variable frequency discriminating meansconnected to the output circuit of said tube, at least one of saidfrequency discriminating means being manually presettable.

9. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interposed between said tone generatorsandoutlet including a common am'- plifier tube having a cathode, controlgrid, and

plate circuit, a manually operable control means for adjusting the gridpotential of said tube to thereby adjust the tube gain, and a pluralityof variable frequency discriminating means connected to the outputcircuit of said tube, at least .f

one of said frequency means being manually presettable.

10. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators andoutlet, including an amplifier tube including a cathode, control grid,and plate oircuit, and manually operable control means for adjusting thegrid potential of said tube to thereby adjust the tube gain, saidcontrol means including a foot pedal and an adjustable resistor operatedthereby, said resistor being operable when completely short circuited toconnect said grid to ground potential.

11. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators and outletincluding an ampliiier, means for adjusting the gain of the amplifier,and frcquency discriminating means connected to the ampliier, saidfrequency discriminating means being operable in response to the gainadjusting means.

12. An electronic musical instrument as defined in claim l1, whereinsaid frequency discriminating means comprises a variable resistor tube,the plate circuit resistance of which is controlled by said gainadjusting means.

13. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of diierent quality, atone outlet, and means interconnecting said tone generators and outletincluding an amplifier, means for controlling the gain of the amplifier,and a plurality of frequency discriminating means corinected to theamplifier, one of said frequency discriminating means being operated inresponse to the gain control means, and means for manually adjusting theother of said frequency discriminating means.

14. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators and outletincluding a common amplifier, means for controlling the gain of theamplifier, a plurality of frequency discriminating means connected tothe amplifier, means for manually presetting at least one of thefrequency discriminating means, and means for operating all of saidfrequency discriminating means in response to the gain controllingmeans.

l5. An electronic musical instrument having a sound producing systemcomprising a plurality of sets of tone generators of different quality,a tone outlet, and means interconnecting said tone generators and outletincluding an amplifier, means for controlling the gain of the amplifier,and frequency discriminating means connected to the ampliiier, saidfrequency discriminating means comprising a plurality of reactanceelements of predetermined different value, and a tap switch forselectively connecting one or more of said elements for operation, andmeans connecting said tap switch to the gain controlling means foroperation therewith.

FRANCIS M. SCHMIDT.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,015,014 Hoschke Sept. 17, 19352,237,105 Miessner Apr. 1, 1941 2,310,429 Hanert Feb. 9, 1943 2,328,282Kock Aug. 3l, 1943 2,429,226 Hanert Oct. 21, 1947

